Forming a barrier between zones in waterflooding

ABSTRACT

In a waterflooding process for oil recovery from oil-bearing earth formations, a high-permeability zone is isolated from a low-permeability zone at the injection well. A water solution of sodium silicate is injected into one zone, a water solution of an activator - a chemical such as ammonium sulfate which causes sodium silicate solution to gel - being injected into the other zone. Preferably, the two solutions are injected simultaneously, the rates of injection being proportional to the pore volumes of the two zones to keep the solution fronts together. Fracturing pressures are avoided.

ilnited States Patent Wagner, Jr. et al.

[451 Apr. 18, 1972 [54] FORMING A BARRIER BETWEEN ZONES IN WATERFLOODING[73] Assignee: Amoco Production Company, Tulsa, Okla.

[22] Filed: Sept. 8, 1970 [21] Appl. No.: 70,557

[52] U.S. Cl ..166/270, 166/269, 166/292 [51] Int. Cl. ..E2lb 33/138,E21b 43/22 [58] Field of Search 166/269, 270, 292, 300, 263, 166/268[56] References Cited UNITED STATES PATENTS 2,207,759 7/1940 Reimers..l66/292 2,208,766 7/1940 Lawton.... ..166/292 2,236,147 3/1941 Lerchet a1... ..166/292 2,330,145 9/1943 Reimers ..l66/292 2,365,039 12/1944Andresen ..166/292 2,402,588 6/1946 Andresen 166/270 2,786,530 3/1957Maly 166/263 3,013,607 12/1961 Bond et a1 ..166/270 X 3,261,400 7/1966Elfrink l 66/ 292 3,285,338 11/1966 Boston ..166/270 3,386,509 6/1968Froning 1 66/300 X Primary Examiner-Stephen J. Novosad Attorney-Paul F.l-lawley and Buell B. Hamilton [5 7] ABSTRACT In a waterflooding processfor oil recovery from oil-bearing earth formations, a high-permeabilityzone is isolated from a 1ow-permeability zone at the injection well. Awater solution of sodium silicate is injected into one zone, a watersolution of an activator a chemical such as ammonium sulfate whichcauses sodium silicate solution to gel being injected into the otherzone. Preferably, the two solutions are injected simultaneously, therates of injection being proportional to the pore volumes of the twozones to keep the solution fronts together. Fracturing pressures areavoided.

9 Claims, No Drawings FORMING A BARRIER BETWEEN ZONES IN WATERFLOODINGWhen water is introduced through injection wells into oilbearing earthformations to displace the oil toward producing wells, a principalproblem is non-uniform flow of the water through zones of differentpermeabilities. It has been proposed that more uniform flow can beobtained by forming a silica gel plug in the more permeable zone.Examples of such processes are described in U.S. Pat. Nos. 2,176,266Malmberg; 2,198,120 Lerch et al.; 2,207,759 Reimers; 2,236,147 Lerch etal.; 2,330,145 Reimers; 2,365,039 Andresen; and 2,402,588 Andresen.

These processes could be greatly improved if two difficulties could beovercome. First, if enough activator, such as am monium sulfate, is usedwith the sodium silicate solution to form a strong gel, the mixture setsso quickly it cannot be displaced as far into the formation as desired.If less activator is used, a longer time is available before the mixturegels, but the final gel is weaker. It is obviously desirable to form astrong gel extending to a great distance from the injection well.Otherwise, the flooding water may flow only a short distance through thelow-permeability zone before flowing vertically into thehigh-permeability zone.

The second difficulty is high temperature. At formation temperaturesabove about 160 F., any mixture of sodium silicate and an activator,which will gel at all, gels so quickly that it cannot be displaced to aneffective distance into a formation at normal injection rates. A methodfor forming a silica gel plug in a high-temperature, high-permeabilityzone would be very useful.

SUMMARY OF THE INVENTION We have found that both difficulties can beovercome by a process in which sodium silicate solution is injected intoone of the zones, preferably the high-permeability zone, and anactivator solution is injected into the other zone, preferably thelow-permeability zone. In this process, when water attempts to flowvertically from the low-permeability zone into the highpermeabilityzone, silica gel immediately forms a barrier between the zones directingthe water back into the lowpermeability zone.

The process is capable of several variations. For purposes ofsimplicity, a process will first be described involving a singlehigh-permeability zone and a single low-permeability zone. Also forpurposes of simplicity, these zones will generally be referred to asloose and tight zones, respectively.

In all variations of the process, the two zones must be isolated fromeach other at the injection well. There are several alternate methodsfor isolation, the preferred one being to set a packer in the wellbetween the zones, a string of tubing extending from the surface throughthe packer.

In the preferred process, after setting the packer between the zones,sodium silicate solution is injected into the loose zone at the sametime ammonium sulfate solution is injected into the tight zone. Therates of injection should be roughly proportional to the pore volumes sothe fronts of both banks move through the formation at substantially thesame speed. For example, suppose the porosity of the loose zone is 40percent and the thickness of this zone is 5 feet, while the porosity ofthe tight zone is percent and the thickness is feet. In this case, theratio of volumes per unit time injected into the two zones should be200:300 (40 X 5:15 X 20). In other words, the rate of injection into thetight zone in this case should be We times the rate of injection intothe loose zone. This will probably mean use of a much higher injectionpressure for the tight zone than for the loose zone. Fracturingpressures must be avoided to prevent the formation of additionalhigh-flow capacity channels.

The lower pressure in the loose zone causes flow from the tight zoneinto the loose zone. When this flow takes place, however, mixing of thesodium silicate and activator solutions approximately at the boundarybetween the zones causes the ing further flow across the boundary.Therefore, the water in- 1 jected into the tight zone is forced toremain in the tight zone until it reaches a point beyond the silica gelbarrier at the boundary with the loose zone. As soon as this point isreached, mixing of the two solutions at the boundary forms more silicagel to extend the disc-shaped silica gel barrier between the two zonesforcing the water to flow even farther in the tight zone. This actioncan continue to the producing well if desired.

It will be apparent that in this process high temperatures have noadverse effects. Silica gel forms only where the two solutions mix andrapid formation of a silica gel barrier is desired. In this process, thehigh temperature simply accelerates formation of the barrier. This is anadvantage. Thus, in our process, higher temperatures provide beneficialeffects rather than adverse effects.

It is not necessary to 'add sodium silicate and activator to all thewater injected into the two zones. Once a silica gel barrier has beenformed between the zones, this barrier prevents vertical flow of plainwater free from gel-forming chemicals. Therefore, the chemicals may beadded only to the leading portions of water injected into each zone.This portion may be as little as about 5 percent of the total volume tobe injected into any zone, but preferably should be about 10 percent.Even larger amounts, such as about 20 percent, may be used in case thereis some uncertainty regarding the pore volumes of the two zones. Use ofthis technique greatly reduces the cost of the process, although sodiumsilicate and its activators are both low-cost chemicals.

The concentration of the silica portion of the sodium silicate in itswater solution should preferably be about 6 percent by weight, butconsiderable variation from this value is possi ble. Since a ratherstrong gel is desired, the silica concentration in the mixture with theactivator solution should be at least about 3 percent. This requires aconcentration of about 6 percent in the silicate solution since theaverage mixture of silicate and activator solutions contains about 50percent of each. To avoid high costs, an upper limit of about 20 percentsilica in the silicate solution should be observed. In most cases, aconcentration in the range from about 6 to about 8 percent is preferred.

The ratio of silica to sodium oxide in the silicate can vary withinlimits from about 1.521 to about 4:1 by weight. Preferably, the ratioshould be from about 3: l to about 3.5: l

Potassium silicate can be used in place of sodium silicate, although thegreater cost of potassium silicate limits its use. Some of the mixturesof sodium and potassium silicates are sometimes preferred because of thelow viscosities of their aqueous solutions.

Aluminates and borates can be substituted for the silicates. Again,however, their greater cost limits their use. Still other settableliquids will occur to those skilled in the art.

Many chemicals can serve as activators. An activator can be defined as achemical which causes a sodium silicate solution to gel. It should benoted that the gel is not simply a precipitate. Heavy metal ions, suchas calcium, can cause precipitation of a water-insoluble heavy metalsilicate. This is not a gelatinous precipitate. Therefore, an activatorfor our purposes should be defined as a chemical which causes theformation of a gelatinous silica gel when added to a sodium silicatesolution.

Most activators are acidic, water-soluble chemicals which lower the pHof the silicate solution to a point at least below about 11. Preferably,the pH should be lowered to a level between about 9 and 10. Mostammonium salts of strong acids are effective. Examples include ammoniumsulfate, ammonium phosphate, ammonium bicarbonate, ammonium nitrate,ammonium chloride, and the like. The acids themselves can also be used.These may include phosphoric, sulfuric, acetic, and hydrochloric acids.Even some of the basic salts can be used. An example is sodiumbicarbonate. The preferred activator is ammonium sulfate, principallybecause of its low cost. The other most common and widely usedactivators are formation of silica gel at this boundary substantiallypreventdiammonium hydrogenphosphate, ammonium bicarbonate,

phosphoric acid, sulfuric acid, acetic acid, and hydrochloric acid.

When the pH of a sodium silicate solution has been lowered to a valuebelow about 11, the presence of substantially neutral salts, such assodium chloride, may affect the amount of silica gel which forms as wellas the rate of gelation. Thus, even some of the neutral salts might beconsidered to be at least secondary activators since they have someeffects on the gelation of sodium silicate solutions which contain aprimary activator. When reference is made to activators herein, however,only the primary activators are intended, such as the ammonium salts,acids, and the like, which cause gelation of sodium silicate solutionswithout the aid of other chemicals.

The concentration of activator in the activator solution depends to someextent on the specific activator which is used. Most of the commonactivators, however, have at least approximately equal effects in equalconcentrations. Activator concentrations in solutions injectedseparately from the sodium silicate solution should have a concentrationof at least about 4 percent. Preferably, the concentration should be inthe range from about 8 to about 10 percent by weight. Even higherconcentrations may be used, particularly if the concentration of sodiumsilicate in the other solution is low. It is generally advisable to usea higher concentration of the less expensive activator in order topermit use of a lower concentration of the more expensive sodiumsilicate. A concentration of more than about percent is rarely advisablebecause of the higher cost. As explained above, these concentrations areabout twice the values desired in the mixtures with silicates since whenthe two solutions mix, the average concentrations of both activator andsilicates in the mixtures will be about half the values in the separatesolutions.

If three or more zones are present with vertical flow possible betweenadjacent zones, then the silicate and activator solutions should beinjected into alternate zones. In this way, a middle zone carryingsilicate solution is in contact with zones above and below carryingactivator solution and vice versa.

While it is preferred to inject the silicate and activator solutionssimultaneously into the separate zones, it is possible in some cases toinject these solutions in sequence. If the solutions are injected insequence, it is best to inject the silicate solution first into theloose zone. The silicate solution in this case should be lightlyactivated before injection. That is, a small amount of activator, suchas about /2 to 1 percent by weight of ammonium sulfate, should be addedbefore the solution is injected. The volume of lightly activatedsilicate solution injected should be at least about 20 percent of thepore volume of the loose streak and preferably at least about 50percent. A lightly activated silicate solution is defined as onecontaining enough activator to cause at least a slight formation of agelatinous precipitate but insufficient to gel the entire solution.

After the silicate solution is injected into the loose zone, anactivator solution should then be injected into the tight zone. Whereverthis activator solution contacts the silicate solution in the loosezone, a silica gel barrier forms to prevent entry of the flooding waterinto the loose zone.

As the activator solution flows through the tight zone, it displaces oiland water already in the formation toward the producing well. This oiland water continually tries to flow into the permeable zone where thepressure is lower and resistance to flow is less. If the displaced watercontains as much as about 4 or 5 percent salt, which is frequently thecase, this salt water will catalyze gelation of the slightly activatedsilicate solution, as previously explained. The resulting gel blocksflow of displaced oil and water into the more permeable zone. This isthe reason for slightly activating the silicate solution beforeinjection if the solutions are not to be injected simultaneously intothe separate zones. If the displaced water and oil are permitted to flowinto the silicate-filled zone without formation of a gel, this water andoil dilutes and displaces the silicate solution, so it is no longereffective to form a gel with the activator solution when the activatorsolution reaches the point at which the displaced water and oil enteredthe loose zone. lf the silicate solution is lightly activated, however,the naturally occurring brine can trigger gelation of the solution andprevent entry of oil and water into the loose zone.

It will be apparent from the above description that there is littlechance for loss of activator solution in the process where silicatesolution is injected first into the loose zone and activator solution isthen injected into the tight zone. Therefore, the volume of activatorsolution in this process may be as little as 2 or 3 percent of the porevolume of the tight zone expected to be flooded. Preferably, however,the activator solution volume should be at least about 5 percent of thispore volume in this process. The small volume of activator solution isthen followed by ordinary flooding water substantially free fromactivators. In this type of process, there is little point in using avolume of activator solution greater than about 20 percent of theportion of the pore volume of the tight zone expected to be flooded.

Many variations of the process are possible using sequential injectionof silicate and activator solutions, one into the loose zone, and oneinto the tight zone. For example, the zones may be first isolated at theinjection well and the lightly activated silicate solution injected intothe loose zone or streak. The isolating means can then be removed,activator solution being injected into both loose and tight zones. Assoon as the activator solution enters the loose zone, a silica gel plugforms to prevent further entry of activator solutions into the loosezone except through the tight zone. If desired, s small batch of waterfree from activator may be injected after the silicate solution todisplace the silicate solution a short distance away from the injectionwell before the silica gel plug forms. This permits higher injectionrates into the injection well since water can enter the loose zone atthe well being diverted into the tight zone only at some distance backin the formation away from the well.

If more than a single loose zone is present and it is desired to injectthe solutions in sequence, a straddle packer can be used to fill each ofthe loose zones with a lightly activated silicate solution. Activatorsolution can then be injected into all zones. The formation of silicagel plugs in the loose zones near the well quickly forces the activatorsolution into the tight zones.

This invention will be better understood from the following example. Anoil-bearing fonnation is made up of two zones. The upper zone is 5 feetthick, has an average porosity of about 40 percent and an averagepermeability of about 800 millidarcys. The lower zone is about 20 feetthick, has an average porosity of about 15 percent, and an averagepermeability of about 200 millidarcys. There are no unconforrnities,shale streaks, or the like, at the level of contact between the zones,the vertical permeability at this level being about 200 millidarcys.

It is obvious that if both zones are flooded without isolation, waterwill streak through the high-permeability zone from the injection welland will reach a producing well long before much of the oil is displacedfrom the low-permeability zone. Therefore, it is decided to use ourprocess.

A packer is set between the zones. An open-ended tubing extends throughthe packer. Two streams of flooding water are injected simultaneouslydown the well. One flows down the annular space between the tubing andcasing and into the upper zone of the formation. The other stream flowsdown the tubing and into the lower zone of the formation.

The stream of water flowing into the upper zone of the formationcontains sodium silicate having a ratio of silica to sodium oxide ofabout 3.5 :l. The concentration of silicate is sufficient to provide asilica content of about 6 percent by weight. This solution is injectedat a rate of about barrels per day (42 U. S. gallons per barrel).

The stream of water flowing into the lower zone of the formationcontains about 8 percent by weight of ammonium sulfate. This solution isinjected at a rate of about barrels per day.

After injecting the silicate and ammonium sulfate solutions into the twozones for about 40 days, ordinary flooding water without silicate oractivator is injected into the two separate zones, the rates ofinjection being the same as when the silicate and activator solutionswere being injected.

After about months from the time of starting the process, water breaksthrough into a producing well about 420 feet from the injection well.Breakthrough of water from both the high-permeability andlow-permeability zones occurs substantially simultaneously since therates of injection into the two zones have been proportional to the porevolumes to be filled with flooding water and there has been little ifany cross-flow between them.

As previously noted, many variations of our process exist. For example,it may be advisable to inject fresh water ahead of the silicate solutionin all cases in order to avoid possible loss of silicate because ofprecipitation by calcium ions, or the like. Other variations will beapparent to those skilled in the art. Therefore, we do not wish to belimited to the descriptions given above but only by the followingclaims.

We claim:

1. A method for waterflooding an oil-bearing underground earth formationhaving ahigh-permeability zone in contact with a low-permeability zoneand penetrated by an injection well and a producing well, said methodcomprising,

isolating the high-permeability zone from the low-permeability zone atthe injection well, and

separately injecting two aqueous solutions into said zones,

one solution containing a silicate selected from the group consisting ofsodium silicate, potassium silicate and mixtures of these two silicates,

the ratio of silica to metal oxide in the silicate being between about1.5:1 and about 4: l and the concentration of silicate being sufficientto provide a silica concentration between about 6 and about percent byweight,

the other solution containing an activator capable of causing saidsilicate solution to gel, the concentration of said activator being fromabout 4 to about 20 percent by weight,

one solution being injected into said high-permeability zone and theother solution being injected into said lowpermeability zone atpressures insufficient to fracture the formations,

whereby mixing of said solutions causes the formation of a silica gelbarrier between said zones to inhibit flow of flooding water from onezone to the other.

2. The method of claim 1 in which the two solutions are injectedsimultaneously into the two zones, the rates of injection beingsubstantially proportional to the pore volumes of the portions of thezones expected to be flooded.

3. The method of claim 2 in which said silicate is sodium silicate,

the ratio of silica to sodium oxide is between about 3:1 and about 3.5:l

the silica concentration is between about 6 and about 8 percent byweight,

the activator is selected from the group consisting of ammonium sulfate,diammonium hydrogen phosphate, ammonium bicarbonate, phosphoric acid,sulfuric acid, acetic acid, and hydrochloric acid, and

the concentration of said activator is from about 8 to about 10 percentby weight.

4. The method of claim 3 in which said activator is ammonium sulfate.

5. The method of claim 2 in which the volume of each solution is fromabout 5 to about 20 percent of the portion of the pore volume expectedto be flooded in the zone into which the solution is injected, and

said solutions are driven through the formation by ordinary floodingwater substantially free from silicates and activators.

6. The method of claim 1 in which said silicate solution is injectedfirst into said high-permeability zone and the activator solution islater injected into said low-permeability zone,

said silicate solution, before injection, being lightly activated tolower the pH of the silicate solution to a value below about 11 and toform a slight precipitate of silica gel.

7. The method of claim 6 in which said silicate is sodium silicate,

the ratio of silica to sodium oxide is between about 3:1 and about3.5:1,

the silica concentration is from about 6 to about 8 percent by weight,

the activator in said silicate solution is present in a concentration ofabout 1 percent by weight,

the activator in the activator solution is present in a concentrationfrom about 8 to about 10 percent by weight,

said activator in both cases being selected from the group consisting ofammonium sulfate, diammonium hydrogen phosphate, ammonium bicarbonate,phosphoric acid, sulfuric acid, acetic acid, and hydrochloric acid.

8. The method of claim 7 in which said activator is ammonium sulfate.

9. The method of claim 6 in which the volume of said silicate solutionis from about 20 to about 50 percent of the pore volume of the portionof said high-permeability zone expected to be flooded,

the volume of said activator solution is from about 5 to about 20percent of the pore volume of the portion of said low-permeability zoneexpected to be flooded, and

said activator solution is followed by ordinary flooding watersubstantially free from silicates and activators.

2. The method of claim 1 in which the two solutions are injectedsimultaneously into the two zones, the rates of injection beingsubstantially proportional to the pore volumes of the portions of thezones expected to be flooded.
 3. The method of claim 2 in which saidsilicate is sodium silicate, the ratio of silica to sodium oxide isbetween about 3:1 and about 3.5:1, the silica concentration is betweenabout 6 and about 8 percent by weight, the activator is selected fromthe group consisting of ammonium sulfate, diammonium hydrogen phosphate,ammonium bicarbonate, phosphoric acid, sulfuric acid, acetic acid, andhydrochloric acid, and the concentration of said activator is from about8 to about 10 percent by weight.
 4. The method of claim 3 in which saidactivator is ammonium sulfate.
 5. The method of claim 2 in which thevolume of each solution is from about 5 to about 20 percent of theportion of the pore volume expected to be flooded in the zone into whichthe solution is injected, and said solutions are driven through theformation by ordinary flooding water substantially free from silicatesand activators.
 6. The method of claim 1 in which said silicate solutionis injected first into said high-permeability zone and the activatorsolution is later injected into said low-permeability zone, saidsilicate solution, before injection, being lightly activated to lowerthe pH of the silicate solution to a value below about 11 and to form aslight precipitate of silica gel.
 7. The method of claim 6 in which saidsilicate is sodium silicate, the ratio of silica to sodium oxide isbetween about 3:1 and about 3.5:1, the silica concentration is fromabout 6 to about 8 percent by weight, the activator in said silicatesolution is present in a concentration of about 1 percent by weight, theactivator in the activator solution is present in a concentration fromabout 8 to about 10 percent by weight, said activator in both casesbeing selected from the group consisting of ammonium sulfate, diammoniumhydrogen phosphate, ammonium bicarbonate, phosphoric acid, sulfuricacid, acetic acid, and hydrochloric acid.
 8. The method of claim 7 inwhich said activator is ammonium sulfate.
 9. The method of claim 6 inwhich the volume of said silicate solution is from about 20 to about 50percent of the pore volume of the portion of said high-permeability zoneexpected tO be flooded, the volume of said activator solution is fromabout 5 to about 20 percent of the pore volume of the portion of saidlow-permeability zone expected to be flooded, and said activatorsolution is followed by ordinary flooding water substantially free fromsilicates and activators.